Microsoft doesn't provide previews for Dashboard Themes on Live, so this forces everybody to buy themes blind, but today I found DashboardThemes.com which has a preview of all the 360's purchasable Dashboard Themes.
Microsoft does provide previews for some Marketplace content, but they need to give full previews for all content when it goes live on Marketplace otherwise there are always going to be irritated customers who bought something and didn't get what they expected.